Neutron meter



Patented May 9, 1950 UNITED S 'I' E N T NEUTRON METER ApplicationNovember 30, 1949, Serial No. 130,092

7 Claims. 1

This invention relates to the measurement of the intensity of neutronradiation and is more particularly concerned with improvements in thattype of neutron radiation meter which relies upon the ionizationproduced by a charged particle emitted by certain neutron sensitivematerials upon the absorption of a slow neutron.

As used in this specification and in the appended claims, the followingterminology is defined as indicated below:

Thermal Neutrons (slow neutrons) -Neutrons having a substantiallyMaxwellian number-em ergy distribution characteristic about an energyvalue equal to kT, where k is a constant and T is the temperature indegrees Kelvin. (kT=0.025 electron volts at 15 C.)

Fast Neutrons-Neutrons having an average kinetic energy greater than100,000 electron volts.

Intermediate N eutrons-Neutrons having an average kinetic energy in therange between that of fast neutrons and that of thermal neutrons.

Fission-The splitting of an atomie nucleus, upon the absorption of aneutron, into a plurality of fragment-,s of greater mass than that of analpha particle, the splitting being accompanied by the release of energyand a plurality of neutrors.

Fissionable--Having the ability to undergo fission upon the absorptionof a slow neutron. Thermal Dfiusion Length of a Material (L) The squareroot of the thermal difiusion area.

Slowing Down Length of a Material (LD-The square root of the neutronage.

Moderator-A non-gaseous material for which the ratio is greater than 10,wherein 5 is the average loss in the logarithm of the energy of a fastneutron per elastic collision within the material, is the slow neutronelastc scattering cross section per atom of the material, and 6a is theslow neutron absorption cross section per atom of the material.

Because of the fact that neutrons are not charged, and therefore, do notproduce any ionization directly, in order to detect and measure theintensity of neutron radiation, it is necessary to measure theionization produced by a charged particle which results from, or isproduced by, the primary neutron radiation. One well known device ofthis character employs a material, such as boron or lithium, which has ahigh cross section for a reaction involving the absorption of a slowneutron and the immediately subsequent emission of a charged particle.In such devices, the boron or lithium containing material may be ingaseous form and itself be used as the gaseous ionizing medium, or itmay be in the form of a coating extending adjacent the gaseous ionizingmedium. One form of the latter type of neutron meter is illustrated inU. S. Patent No. 2,288,718, entitled Device for measuring the intensityof a radiation of slow neutrons by means of ionization chamber," issuedJuly 7, 1942, to H. I. Kallmann.

A serious problem in connection with the abovedescribed types of neutronmeter arises from the fact that a neutron absorbing material cannot beplaced within a neutron flux region without simultaneously efiecting adecrease in the neutron ux in the neighborhood. Since the neutronsensitive material of the meter has a high absorption cross section forneutrons, it acts as a neutron sink, thereby depressing the neutron fluxright at the point of detection and decreasing the efiectiveness of theneutron sensitive material. This circumstance has heretofore seriouslyreduced the sensitivity and eiiiciency of neutron meters of this type.

Furthermore, prior art neutron meters of this type sufier from thedisadvantage that their sensitivity to detect neutrons varies sharplywith the energy of the neutrons, that is, they are much more sensitiveto the slower neutrons. In cases where it is desired to measure theradiation of fast neutrons, or the total radiation of fast and slowneutrons together, this represents a serious disadvantage.

Accordingly, it is an object of the present invention to provide animproved neutron meter characterized particularly by an increasedsensitivity over neutron meters heretofore employed.

Another object of the present invention is to provide a neutron meterhaving means to counteract the depression of neutron fiux normallyresulting from the presence in the meter of material having a highabsorption cross section for neutrons.

A further object of this invention is to provide a neutron meter whichis adapted to the measurement of fast, intermediate, or slow neutrons,or the combined radiation of neutrons of all energies.

Another object of the invention is to provide a neutron meter theensitivity of which varies only slowly with neutron energy so that ithas comparable sensitivity to slow and to fast neu trons.

Still another object of the invention is to provide a neutron meterhaving a large ration oi neutron to gamma ray sensitivity so that. itsreadings are subject to minimum error' from the presence of gamma fluxesin addition to the neutron flux which is being measured.

Applicants accomplish the above advantages; and overcome thedifiiculties inherent in prior art devices, in part, by splitting up theregioncontaining neutron' sensitive material into a. plu rality ofspaced individual re'gions* of large area, thereby providing a highsurface to Volume ratio. In order to overcome the effect ofself-absorption in the neutron sensitive regione, there isdisposedintermediate to each of said regions, and` pret erably also outside ofeach of said regions, a layer of moderator material, wherein the neutronflux, due to the unique nuclear characteristics of' such moderatormaterial, builds up to; a high intensity; Applicants have' found* thatthe thickness of each layer of moderator' material, for` mostadvantageous efiect, shouldhave a critical* rela tionship to the thermaldifiusion length of such moderator material, as will be described indetail hereinafter. Applicants have also found that for greatest'effectiveness, a critical relationship should bemaintained between thesum of the thicknesses of the moderatorlayers and the. slow ing downlength of the moderator material, as' will be fully explainedherein'after.

Other objects` and advantages of the present invention will becomeapparent from the accompanying description when taken in' connecton withthe accompanying' drawings.

In, the drawings',

Fig. 1 is a combined sectionalelevation View of' the structure, andschematic wiringdiagram of the circuit, of' the present invention Fig.2' is a top plan view of` the structural portioncf Fig. 1.

Referring now' to the' drawings, there is pro vided an outermetalchamber l' and aninner` metal ch'amber` 2. Chambers' l and 2 aregastight and are insulated' from each' other'by tubular glass insulators&and 4; Insulators 3" and 4, in' conjunction With tubuiar'metallicmember &serve as a support for'theinner chamb'er'z'. Cham bers l and 2are' preferabl'y, although-not necessarily, circular incross-section*asindicated in-Fgi 2; The spa-ce within chamber 2 isfilled with' a' moderator material fl, and the' space between chamber 2and chamb'er l` is'filled' to a high pres'- sure with a gaseous materialE, the properties and characteristics of materials 6 and "F to bemorefully discussed hereinafter. Metal filling tube 8 and its associatedValve= are provided for the insertion and removal of material 6; andmetalfilling tube lil and its associated va-lve' H are provided' for`the' insertion and' remova'lof moderator material 'L surrounding the'-outer chamber I on all sides, there is` preferably dis posed anadditional layerof' moderatorma-- terial T. v i

A source of 'direct voltage, such as b'attery" l-2 is provided, thenegative terminal of the-battery` being' connected' to chamber I' by wayof tubuiar member 5, and thepositive terminal of the bat-- tery beingconnected to gro'und Ghamber 2 is also connected to ground: Chamber 2-is also; connected to ground through tube o and re-- sistance 3-. Ametalflguard'ring i d is'sealed between insulators 3' and a for-thepurposeofp'reventing creepage of charge along these insulators. Atubular metal shield !5, surrounding and concentric with filling tubeID, is provided, this shield terminating at, and being connected to,guard ring l. Ring i and shield !5 are grounded, as' shown If' desired,the insu- Iators 3 and !i could bei made: integra liand the guard ring Mreplaced by a grounded conducting 'band placed both inside and outsideof the integral` insulator. i

A conventional D. C. amplifier !6 is connected to receive the voltagedeveloped across resistance 13' as its input signal. The output of theD. C. amplifier ta feeds. any suitable indicating device or recordinginstrument, indicated as ammeter ll.

It will be apparent that the space between chambers. l and 2 willoperate in the manner of a conventional ionization chamber, with thechambers i and 2 constituting the oppostely charged electrodes; The'external circuit constitutes' a conventional ampliiying circuit normallyassociated with` an ionization chamber The gaseous* material 8 fillingthe space be*- tween chamber i and ch'amber` 2 is one which contains anucleus with` a large* slow' cross section and thereupon immediatelyreleasing' a charged' parti'ole. Li and 13 are examples of such nuclei'involving* the absorption of a slow neutron and the ernission' of'anal'p'ha particle: The. gaseous material 6 is preferably* borontri`-fluoride; gas at a highpressure Since naturally occurring' lithiumcontains; only about- 10% Li and'. natura-Hy occnrring boton contains.only about, 20 13- increased efiiciency can be realized by empl'oying a:gaseous material: enriched in the neutron: sensitive isotope, as;discussed: in' the; above' referred to; Kallmann; patent;

Gaseous compounds containing fissionable nucle,. such as 11 Fa or; Uwould, of' course also: be suitable? for use as the gaseoussmaterial e,the charged fission fragmcnts released in the fission process serving,in such case to ionize the. gas..

Although the: gaseous material 6. preferably contains within itself..theneutron sensitive, material this, of course, is; not. necessary, it.only being required. that; there be provided: within chamber i any gas,and that neutron sensitive material be somehow disposed. throughout onadjacentte the; gas. Thus; if desired, theneutren sensitive materialcould be in: solid form disposed. on a. plurality of, supporting stripswithin chamber i for example, o: disposedon. the inner surface otchamber l and the oter surface of chamber 2.

The material 'i within chamber 2 should be characterized by a highneutron Scattering cross section, a high value of" a low neutronabsorption cross section, and should be fairly dense. Non gaseoushydrcgen-containing compounds, such as water, paraffim or: hexane,censtitute good moderators andi are eminently suitable for this'purpose., For reasons: which will become evidenthereinater. para-nin isideal.. for use as material?, and; its use-is: preferred in thepresentinvention. r

In operation, fast neutrons and intermediate energy neutrons enteringthe. meter: are slowed down' by collision' with nuclei' of the'moderator" material 'l to' a low energy at which they-'are' readilyabsorbed'in material G Theseneutrons; as wellas already-thermalneuti'ons entering'the m'e-ter, are capture'd in theneutron sensitive material- (i: The resultant' emissibn of a*- charged particle producesionization of the gaseous material E, which ionization produces aproportional current fiow through resistance !3. This current flow isamplified by amplifier IS, and the amplified value is indicated orrecorded on meter I'I, thereby providing an indication of the overallneutron flux.

Applicants have found that the sensitivity of the meter is highest whenthe thickness (t) of each individual layer of moderator material is ofthe order of twice the thermal difiusion length (L) of this material.Applicants have also discovered that the effectiveness of the moderatormaterial in slowing down neutrons is most pronounced when the summationof the thicknesses of the individual layers of moderator material is ofthe order of the slowing down length (Li) of the material for neutronsof the particular energy level expected to be encountered in anyspecific application.

Parafin is uniquely adapted for use as material 1 because of itsparticular values of thermal diffusion length and slowing down lengthand the particular relationship of such values. Since its thermaldifiusion length is about two-thirds of an inch, the preferred thicknessof each of the individual layers of moderator material is about one andone-third inch, which, of course, is an eminently practicable dimension.The slowing down length of paraffin for neutrons of the energy levelmost frequently encountered in practice (fission neutrons having anaverage energy of about-2 m. e. v.) is about four inches. Thus, thepreferred total thicknss of all the layers of paraflin is not only inthe practicable range, but also turns out in the most usual case to beabout three times the preferred value of the individual thicknesses. Itis evident that this fortuitous circumstance permits the realization ofboth of the preferred relationships by a very simple and practicablestructure utilizing two spaced layers of neutron sensitive gas toseparate three spaced layers of paraflin, as shown in the drawings.

It is understood that all matters contained in the above description areillustrative only and do not limit the scope of this invention as it isintended to claim the invention as broadly as possible in view of theprior art. In particular, it should be understood that as manyadditional alternate layers of neutron sensitive material and moderatormaterial may be provided as is found to be practicable. Also, it shouldbe understood that the meter would be operable, although not assatisfactory, without the surrounding layer of moderator material, thatis, with only one intermediate layer of moderator material.

We claim:

1. A neutron meter comprising a, plurality of spaced regions containinga gaseous boron compound, at least one intermediate region formed of anon-gaseous hydrogen containing compound, and means for measuring theionization of said gaseous boren compound.

2. A neutron meter comprising a plurality of spaced layers containingnuclei capable of emitting a charged particle upon the absorption of athermal neutron, at least one intermediate layer formed of a moderatormaterial, and means responsive to the ionization produced by saidcharged particle, said intermediate layer having a thickness of theorder of twice the thermal diffusion length of said moderator material.

3. A neutron meter comprising a plurality of spaced layers containingnuclei capable of emitting a charged particle upon the absorption of athermal neutron, at least one intermediate layer formed of a moderatormaterial, and means responsive to the ionization produced by saidcharged particle, the total of the thicknesses of said intermediatelayers being of the order of the slowing down length of said moderatormaterial for neutrons of the energy level intended to be measured.

4. A neutron meter comprising a plurality of spaced layers containingnuclei capable of emitting a charged particle upon the absorption of athermal neutron, at least one intermediate layer formed of a moderatormaterial, and means responsive to the ionization produced by saidcharged particle, at least one of said intermediate layers having athickness of the order of twice the thermal diusion length of saidmoderator material, the total of the thicknesses of said intermediatelayers being of the order of the slowing down length of said moderatormaterial for neutrons of the energy level intended to be measured.

5. A 'neutron meter comprising an inner gastight metallic container anda concentric outer gas-tight metallic container, said containers be.-ing insulated from one another, said inner container being filled with anon-gaseous moderator material, said outer container being filled with agas capable of emitting a charged particle upon the absorption of athermal neutron, and an external circuit associated with saidcontainers, said circuit including means for maintaining said containersat different potentials and means for measuring the current flowing insaid external circuit.

6. Apparatus, as claimed in claim 5, further including an additionallayer of moderator material surrounding said outer chamber.

7. Apparatus, as claimed in claim 5, wherein said non-gaseous moderatormaterial is paramn, and said gas is boron trifiuoride.

LYNN H. STAUFFER. THOMA M. SNYDER.

REFERENCES CTED The following references are of record in the file ofthis patent:

Atomic Energy Commission Document, AECD, 1954, 5 pages, March 8, 1948.

